Biaxially oriented polypropylene films are widely used for packaging, tapes and capacitors, since they are excellent in optical properties such as clarity and gloss, mechanical properties such as tensile strength and elongation, moisture barrier properties and electrical properties. The biaxial orientation of film is conventionally achieved by drawing the film lengthwise (the "machine direction") and then, with a tenter, drawing the film in a direction normal to the machine direction, in the "transverse direction."Biaxially oriented polypropylene films are major materials used as dielectrics of film capacitors, but since they are lower in heat resistance as compared to polyester film, another major material, the capacitors obtained by using them are limited to about 85.degree. C. in maximum operating temperature. As for the cause, if the operating temperature becomes high, the dielectric strength peculiar to the polypropylene films drastically declines due to the influence of the amorphous domains of the film and impurities, particularly making them unable to withstand long-term use in some cases.
On the other hand, as electric apparatuses are being downsized, devices are assembled at higher densities and exposed to higher temperatures, and there is a strong demand to use conventional polypropylene film capacitors at higher operating temperatures. Especially for capacitors intended to be mounted on AC circuits, polypropylene films with a feature of small dielectric loss have been used as dielectrics for the necessity of inhibiting heat generation from inside the capacitor devices, but since the temperature of the environment surrounding AC circuits becomes higher, it is now difficult to use polypropylene films for such an application. This is one of the factors tending to inhibit the downsizing of electric apparatuses equipped with polypropylene film capacitors. As for the reason, even when a circuit has been positioned certain distance away from the heat source, or insulated from the heat source by a heat insulator lest the circuit should be heated by the heat source, there is an increasing demand to install such a circuit very close to a heat source without using any heat insulators, to meet the demand for downsizing. This demand is typically seen for the AC circuits used in the illumination stabilizers of street lighting, etc. and the control circuits of motors. To meet this demand, it is necessary that the polypropylene film maintains its performance even at temperatures higher than 85.degree. C., the maximum operating temperature of conventional polypropylene film capacitors, for a longer period of time.
So, polypropylene film used as a dielectric is demanded to be (1) moderately small in mechanical deformation, i.e., heat shrinkage when rapidly heated within a short time, (2) excellent in electrical properties at a high temperatures and (3) small in decline of electrical properties at high temperatures with the lapse of time.
As for the reason for requirement (1), when a capacitor device is produced, the polypropylene film wound with an electrode is generally annealed at a certain temperature, and thereby moderately thermally shrunken for tightening the winding to maintain its shape and to squeeze out the air between film layers, and in this case if the heat shrinkage is too large, the device may be deformed, lowering the capacity of the capacitor, or may be destroyed. If the heat shrinkage is too small, the tightening of the winding may be insufficient, or the dielectric loss may increase to destroy the device when used at a high temperature for a long time.
It is sometimes practiced to impregnate the capacity device with an insulating oil, for inhibiting the corona otherwise generated, the capacity decline and dielectric breakdown of the capacitor in continuous use and the increase of leak current. In such impregnation with an insulating oil, (4) it is necessary to design the film surface and (5) it is necessary to inhibit the dimensional change and swelling caused by the insulating oil, in order to ensure that the insulating oil is uniformly impregnated into the clearances between film layers when the polypropylene film used as a dielectric is wound as a capacitor device.
Furthermore, while the improvement of above mentioned properties is desired to allow the downsizing of electric apparatuses as described above, there is also a more intensive demand for further downsizing the capacitor itself. For this purpose, the electrostatic capacity of the capacitor per unit volume must be increased, and the film as the dielectric must be thinned. This requires the film to have a higher dielectric strength even at room temperature for meeting the above mentioned demand for a thinner film, though the film has been conventionally designed to be thicker to provide an allowance in dielectric properties.
Especially a capacitor intended for a high frequency circuit, using a polypropylene film as dielectric, is required to have higher dielectric properties and electric current resistance.
To solve the problem, Japanese Patent Laid-Open (Kokai) No. 6-236709 discloses a high molecular insulating material low in ash content, of 1 to 10 wt % in boiling n-heptane soluble content, hence excellent in processability, and excellent in dielectric properties in a temperature range from room temperature to 80.degree. C., and it is stated that a material of 90% or more in the isotactic pentad fraction of the boiling n-heptane insoluble portion is preferable.
Japanese Patent Laid-Open (Kokai) No. 7-25946 proposes a propylene polymer of 80 wt % or more, especially preferably 96 wt % or more in boiling heptane insoluble content and 0.970 to 0.995 in the isotactic pentad fraction of the boiling heptane insoluble component, and also proposes a molding obtained by using the polymer.
However, biaxially oriented polypropylene films that are simply high in the isotactic pentad fraction of the boiling n-heptane insoluble portion, as proposed there, are insufficient in dielectric properties at temperatures higher than 85.degree. C. as intended in the present invention, and also in long-term heat resistance of the capacitor device using the film as a dielectric. That is, though the biaxially oriented polypropylene films high in stereoregularity obtained by said conventional techniques are also high in the isotactic pentad fraction of the boiling n-heptane insoluble portion, the isotactic pentad fraction as a film is destined to be low since the isotactic pentad fraction of the n-heptane soluble portion is low. As a result, the stereoregularity is insufficient. In addition, a so-called high crystalline biaxially oriented polypropylene film very high in isotacticity is very poor in film formability since it is insufficient in stereoregularity, and no industrially useful technique has been established to produce a biaxially oriented polypropylene film that is excellent in bot heat resistance and dielectric properties.
As a technique to overcome this disadvantage, Japanese Patent Publication (Kokoku) No. 4-28727 proposes a crystalline polypropylene film excellent in moldability by keeping the isotactic pentad fraction in a range of 0.960 to 0.990 and keeping the total amount of the extracts obtained by sequentially extracting with boiling n-hexane and boiling n-heptane in a range of 3.0 to 6.0%. However, the film is insufficient in isotactic pentad fraction as well as in dielectric properties at high temperatures.
Furthermore, Japanese Patent Laid-Open (Kokai) No. 5-217799 proposes a metallized film capacitor using a high-modulus metallized film obtained by vapor-depositing a metal on a high-modulus polypropylene film with a specific thermal deformation temperature and a specific Young's modulus, and high in crystallinity and good in stereoregularity. However, its stereoregularity is only about 90% at the highest, and the dielectric properties at high temperatures are insufficient.
Moreover, Japanese Patent Laid-Open (Kokai) No. 7-50224 proposes a metallized polypropylene film of 4.0% or less in heat shrinkage at 120.degree. C. in the machine direction and 0.8% or less in the transverse direction. However, the film is at the conventional levels in isotacticity and stereoregularity, and cannot be said to be sufficient in dielectric properties at the high temperatures intended in the present invention, or to meet the-advanced demand in future.